Membrane Distillation Research Articles

Performance prediction model for desalination plants using modified grey wolf optimizer based artificial neural network approach

Desalination represents an effective method for alleviating water scarcity, applying algorithmic techniques to predict the performance of reverse osmosis (RO) desalination plants, Modified Grey Wolf Optimizer (MGWO) based Artificial Neural Networks (ANN) can predict the performance of membrane distillation (MD) equipment. Four experimental inputs are selected: feed salt concentration(35–140 g/h), feed flow rate(400–600 L/h), evaporator inlet temperature (60–80 ℃), and condenser inlet temperature (20–30 ℃). The permeate flux (L/h m2) is selected as the experimental output. Ten prediction models were proposed and compared with the existing models (ANN, WOA-ANN, and GWO-ANN). The results showed that the MGWO-ANN model-5 showed the best regression results: R2 = 99.3 %, mean square error (MSE)= 0.004. This model outperformed the existing ANN model (R2 =98.8 %, MSE=0.060), WOA-ANN model (R2 =99.1 %, MSE=0.005) and GWO-ANN model (R2 =98.9 %, MSE=0.007). Model-5 has a single hidden layer (H=1), 13 hidden nodes (n = 13), 10 search agents (SA=10), and 75 %− 20 %− 05 % dataset division. Its residual error is within acceptable limits (spanning −0.1 to 0.2). Optimizing the number of hidden layer nodes (n) and the number of search agents (SA) can improve the training efficiency and prediction accuracy of the model, and the MGWO-ANN model is capable of more accurately predicting the performance of desalination plants.

A spacer-based approach for localized Joule heating in membrane distillation

Membrane distillation (MD) is a versatile low-temperature separation process used for desalinating saline solutions with high salt rejection rates. Its current drawbacks include low flux and high energy demand. This study presents localized electrically induced heating using ceramic-coated metallic spacers to improve MD performance. We coated Ni-Cr spacers with MgO via electrolytic deposition and calcination, optimizing for a crack-free protective surface. Smaller wire diameter Ni-Cr exhibited superior heating. When a periodic current of 0.2 A cm−2 was applied, permeate flux increased by 15% although energy consumption only increased by 4%. Continuous supply of high-grade electrical energy added no further performance improvement as compared to periodic application. Our work highlights a spacer-based approach for localized Joule heating in MD systems without compromising membrane structure, while exploring coating systems to protect conductive spacers and optimizing schemes for electrically controlled performance.

Bead-Containing Superhydrophobic Nanofiber Membrane for Membrane Distillation.

This study introduces an innovative approach to enhancing membrane distillation (MD) performance by developing bead-containing superhydrophobic sulfonated polyethersulfone (SPES) nanofibers with S-MWCNTs. By leveraging SPES's inherent hydrophobicity and thermal stability, combined with a nanostructured fibrous configuration, we engineered beads designed to optimize the MD process for water purification applications. Here, oxidized hydrophobic S-MWCNTs were dispersed in a SPES solution at concentrations of 0.5% and 1.0% by weight. These bead membranes are fabricated using a novel electrospinning technique, followed by a post-treatment with the hydrophobic polyfluorinated grafting agent to augment nanofiber membrane surface properties, thereby achieving superhydrophobicity with a water contact angle (WCA) of 145 ± 2° and a higher surface roughness of 512 nm. The enhanced membrane demonstrated a water flux of 87.3 Lm-2 h-1 and achieved nearly 99% salt rejection efficiency at room temperature, using a 3 wt% sodium chloride (NaCl) solution as the feed. The results highlight the potential of superhydrophobic SPES nanofiber beads in revolutionizing MD technology, offering a scalable, efficient, and robust membrane for salt rejection.

Enhanced performance of electrospun PVDF membranes modified with rare-earth metal oxides for membrane distillation: A systematic study

In the presented studies, the novel type of separation materials dedicated to membrane distillation was generated and systematically analyzed. Utilizing poly(vinylidene fluoride) (PVDF) electrospun membranes as a base, hybrid organic-inorganic materials were developed to enhance performance. These materials exhibited high hydrophobicity, featuring superhydrophobicity on a micro-scale due to fractal-like structures on the inorganic domain surface. Rare-earth metal oxides (REMO) were implemented for the first time as smart modifiers, covalently attached to the functionalized membrane surface. Three REMO types were selected to investigate their impact on membrane performance in air-gap membrane distillation (AGMD) for the desalination process. Chemical attachment via long enough linkers formed a flexible active surface structure, facilitating transport and separation. Dynamic goniometric studies evaluated membrane wetting stability. Particularly noteworthy was the membrane enhanced with Ho2O3 particles, with high flux and salt rejection rates of 14.41 ± 1.35 kg m−2 h−1 and >99.5 %, respectively. The membrane stability and affinity were further examined using Hansen Solubility Parameters and Pearson's hard–soft acid–base (HSAB) theory. The potential of the hybrid membranes is seen not only in desalination. With the ability to control surface properties by using different silane linkers, oleophobicity/oleophilicity can be achieved, and practical use in wastewater treatment or oil/water separation is possible.

Pondering performance of hybrid microfiltration and membrane distillation (MF-MD) process for purifying chromium-contaminated groundwater: Optimization, fouling control, and long-term operation

Nowadays, a hazardous compound such as Chromium (Cr) in groundwater is an emerging issue worldwide due to its environmental and health impacts. Membrane distillation (MD) is a promising technique to purify Chromium-contaminated groundwater (CCG). Assisted microfiltration (MF) is lodged to overcome a major problem in MD, specifically membrane fouling. In this work, a hybrid MF-MD process is proposed with fouling control incorporated into long-term operation. Derived from the results, mixed cellulose ester with 0.1 μm of pore size (MCE0.1) at 3-min ultrasonication cleaning is chosen as the optimal parameter in the MF process, because of superior flux recovery ratio (FRR) with negligible flux deficiency of approximately 10 %. Permeate flux and total suspended solids (TSS) rejection of MCE0.1 was achieved around 440 Lm−2h−1bar−1 and > 90 %, respectively. The optimized parameters in MD were selected accordingly: feed temperature of 80 °C, and feed and coolant flow rates at 4 L/min. Eventually, the long-term batch operation was executed by obtaining average flux and permeate conductivity was 7.5 LMH and around 10 μs/cm, respectively. The Cr concentration in permeate was relatively stable around 0.05 mg/L, which meets the wastewater discharge limit of 0.5 mg/L.

PVDF membranes for membrane distillation prepared by VIPS – A combined study of artificial water channel effectiveness and membrane performance prediction

Background A combined VIPS-NIPS technique was used to investigate the potential of self-supporting flat-sheet polyvinylidene fluoride (PVDF) membranes containing amphiphilic I-quartet Artificial Water Channels (AWC) for Direct Contact Membrane Distillation (DCMD). The AWC are formed in situ by self-assembly of HC6 molecules upon contact with water during membrane formation. Methods A Design of Experiment (DoE) was used to investigate the influence of various experimental parameters and their interactions on membrane performance, namely polymer and AWC concentrations in the dope solution, relative humidity and duration of the VIPS treatment. Pure water permeability (PWP), Liquid Entry Pressure (LEP) and DCMD flux were used to characterize membrane performance. A Response Surface Methodology (RSM) was used to evaluate the DoE results and a second order model was fitted. Based on the model predictions several multiple response-optimized (MRO) membranes were prepared. Results The performance improvement of the MRO membranes was 7.0 times for PWP (M-T4) and 3.3 times for MD flux (M-T3) over the average performance of the DoE membranes, while maintaining LEP of 1.9 bar and Salt rejection of 99.9%. In addition, hybrid PVDF/HC6 MRO membranes improved PWP by 3.2 times and MD flux by 1.6 times over pure PVDF MRO membranes. Conclusions DoE in conjunction with RSM enabled the prediction of membrane PWP, LEP and MD flux performance by varying the levels of the experimental parameters within predefined limits. The HC6 additive improves PWP and MD flux without loss of LEP or selectivity. HC6 promotes the formation of porous skin and asymmetric cross-sections with finger-like structures. Probably by affecting diffusion during phase separation due to its amphiphilic properties.

In Situ-Grown Al2O3 Nanoflowers and Hydrophobic Modification Enable Superhydrophobic SiC Ceramic Membranes for Membrane Distillation

Membrane distillation (MD) is considered a promising technology for desalination. In the MD process, membrane pores are easily contaminated and wetted, which will degrade the permeate flux and salt rejection of the membrane. In this work, SiC ceramic membranes were used as the supports, and an Al2O3 micro-nano structure was constructed on its surface. The surface energy of Al2O3@SiC micro-nano composite membranes was reduced by organosilane grafting modification. The effective deposition of Al2O3 nanoflowers on the membrane surface increased membrane roughness and enhanced the anti-fouling and anti-wetting properties of the membranes. Simultaneously, the presence of nanoflowers also regulated the pore structures and thus decreased the membrane pore size. In addition, the effects of Al2(SO4)3 concentration and sintering temperature on the surface morphology and performance of the membranes were investigated in detail. It was demonstrated that the water contact angle of the resulting membrane was 152.4°, which was higher than that of the pristine membrane (138.8°). In the treatment of saline water containing 35 g/L of NaCl, the permeate flux was about 11.1 kg⋅m−2⋅h−1 and the salt rejection was above 99.9%. Note that the pristine ceramic membrane cannot be employed for MD due to its larger membrane pore size. This work provides a new method for preparing superhydrophobic ceramic membranes for MD.

Optimizing fouling mitigation in membrane distillation by controlled microbubble size and concentration

Fouling control strategies play a crucial role in membrane distillation (MD) process, primarily due to the significant limitation of fouling during the scaling-up process. The utilization of microbubbles (MBs) aeration has exhibited promising potential in mitigating membrane fouling when treating high salinity wastewater. Aeration can introduce gas-liquid two-phase flow on the membrane surface, thereby increasing the membrane surface shear force. The mechanism of aeration technology for controlling fouling includes the following points: introduction of gas-liquid two-phase flow; physical substitution of concentration polarization layer; pressure pulses caused by bubbles flowing through the membrane surface and increasement of the cross flow velocity. This research aims to optimizing the effect of air MBs with different concentrations and sizes introduced in the feed side on membrane fouling control, we creatively used the method of releasing pressure gradiently to achieve size regulation of MBs within a certain range for the first time, firstly regulated the generation of MBs by manipulating gas flow rate and release pressure. Under the optimal condition of 40 mL/min and 0.6 MPa, it will yield concentration of 1.58 × 104/mL and D50 Sauter size of 64.69 μm MBs with pure water permeance flux of 12.32 kg/(m2·h). It was observed that higher gas flow rates primarily increased MBs concentration, while lower pressures predominantly reduced MBs size through the method of inline imaging. Experimental results indicated that introduction of MBs can slightly improve the temperature polarization coefficient and mass transfer coefficient under different operating modes. Consequently, further investigations focused on the influence of MBs characteristics on different fouling control. All conditions reach 99 % rejection for inorganic salt and HA. This study provided empirical evidence that higher concentration and larger size of MBs intensify the gas-liquid two-phase flow disruption and exhibit a better shear effect on scaling phenomena, which yield advantageous outcomes in terms of augmenting permeance flux and water vapor production, with permeance flux increased from 40.12 % to 55.42 % for inorganic fouling and increased from 68.35 % to 80.15 % for inorganic and HA fouling. These results were confirmed via membrane surface using scanning electron microscopy and energy dispersive spectroscopy. In summary, this research aided in identifying appropriate operating conditions for mitigating inorganic and composite scaling by implementing air-infused MBs in direct contact membrane distillation (DCMD), which provides scientific and technical guidance for controlling fouling in MD in practical applications.

A Janus membrane with silica nanoparticles interlayer for treating coal mine water via membrane distillation

The use of membrane distillation (MD) technology to treat coal mine water content has great potential, and the Janus membrane is very resistant to scaling and wetting during MD. In this work, SiO2 nanoparticles were electrostatically adsorbed onto the surface of a polyvinylidene fluoride (PVDF) membrane, followed by the construction of a polyamide (PA) layer through interfacial polymerization (IP). The intermediate layer of SiO2 nanoparticles can promote the enrichment of amine monomers during the subsequent IP process as well as slow its entry into the organic phase to regulate the PA layer. The addition of SiO2 nanoparticles was increased from 0 to 0.10 %, and the thickness of the polyamide layer was also reduced from 225.1 nm to 102.3 nm. Compared with the original membrane vapor flux of 40.9 ± 1.0LMH, the modified membrane vapor flux slightly increased to 46.2 ± 0.9LMH, but excessive SiO2 nanoparticles blocked the membrane pore, resulting in a decrease in flux. MD experiments show that the Janus membrane is wettability and scaling resistance when passing through the salt solution containing sodium dodecyl sulfate (SDS) surfactant or simulated wastewater as feed. At the same time, the interface thermodynamics of the simulated wastewater and the membrane are calculated to understand the anti-scaling mechanism of the membrane, offering a theoretical foundation for the application of real mine water.

Direct deposition of Ti/MgF2 photoabsorber on PTFE membrane with enhanced robustness via plasma pretreatment for photothermal membrane distillation

Deposition of metal-dielectric hybrid composite on the polymer based membrane is important for the electrochemical, photochemical and photothermal reactions on the membrane surface. The hydrophilic reactants such as catalysts and photothermal materials are not easy to be deposited directly on the hydrophobic membrane surface, so in most cases, binder material has been adapted to stably combine them. In this study, commercial PTFE membrane and Ti/MgF2 photothermal layers were combined without any binder. Ar plasma pretreatment was applied to a membrane surface before depositing optimized Ti/MgF2 stacks to fabricate a mechanically robust photothermal membrane. The successful fabrication of Ar plasma pretreated Ti/MgF2 photothermal membrane was demonstrated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscope (AFM) characterization. Through Ar plasma pretreatment, an increase in Ti/MgF2 photothermal membrane heating temperature of 40.5 % was achieved under 1 sun condition compared with unpretreated membrane. This membrane showed improved performance and reduced energy loss in solar desalination. Utilizing these membranes in a membrane distillation system, water flux increased by up to 26.9 %, performance ratio (PR) increased by 18.9 % and thermal efficiency improved by 21.9 %. After 24-h stress test, the Ar plasma pretreated Ti/MgF2 photothermal membranes demonstrated durability and maintained thermal efficiency.

Investigation of membrane fouling behaviors triggered by different characteristics of anaerobic digestion effluent in membrane distillation

This study investigated the membrane fouling behaviors triggered by different characteristics of anaerobic digestion (AD) effluents in the operation of 851-h membrane distillation (MD). The AD effluents with different concentration factors (CF) were used as the feed separately. Results illustrated a stable normalized water flux in the treatment of raw AD effluent (CF = 1) over the 851-h MD experiment, showing negligible membrane fouling. While the sharp decreases in normalized water flux occurred when treating the concentrated AD effluents. The membrane fouling mechanism tended to change from inorganic scaling to organic fouling when the CF value increased from 1 to 3, while the combination of organic and inorganic fouling dominated the membrane fouling when CF was 5. Inorganic fouling was further intensified when the CF value increased to 10. The regression analysis demonstrated that the complete blocking model was the dominant development mechanism for CF = 3 treatment. The membrane fouling mechanism of CF = 5 treatment switched from the cake filtration model in the intermediate phase to the standard blocking model in the final phase. Additionally, there was a transition from the complete blocking model to the standard blocking model for CF = 10 treatment. The study may provide an in-depth knowledge of membrane fouling control during the actual AD effluent concentrating process.

Optimization of laser-induced graphene membrane for simultaneous photo- and electro-thermal membrane distillation

To optimize the Laser-Induced Graphene (LIG) Janus membrane, this study investigated the effects of membrane pore structure, polydimethylsiloxane (PDMS) coating sequence and addition of silver (Ag) nanoparticles on membrane distillation (MD) performance. This study aimed to enhance the photothermal characteristics of graphene while using the intrinsic electrical conductivity for simultaneous photo- and electrothermal MD. Operating at the same photo- and electro-thermal power input, the LIG Janus membrane made by treating the membrane face with smaller pores (i.e., shiny side) gave an improved flux performance of up to 53.6% and a decrease in specific energy of 35.4% compared to that by treating the membrane face with larger pores (i.e., dull side). The effect of the PDMS coating sequence also depended on the pore structure. For the face with smaller pore structures, coating PDMS before laser irradiation (PDMS-BLSS) gave a flux improvement of up to 24.5% and a decrease in specific energy of 19.7%, compared to coating PDMS after laser irradiation (PDMS-ALSS). As for the face with larger pore structures, coating PDMS before laser irradiation (PDMS-BLDS) resulted in a flux reduction of up to 20.8% and an increase in specific energy of 27.1%, compared to coating PDMS after irradiation (PDMS-ALDS). The LIG Janus membranes embedded with Ag nanoparticles led to improved photothermal heating properties, improving flux by 43.1–65.8% and decreasing specific energy by 15.2–30.5% while maintaining similar electrothermal heating properties. Carrying out simultaneous photo- and electro-thermal MD indicates that only the Ag-doped Janus LIG membrane gave a synergistic effect whereby the flux of the combined heating mode was higher than the summation of the fluxes obtained when operating in the individual heating modes.

Advancing Surface‐Enhanced Electrospun Nanofiber Membranes: Customizing Properties for Enhanced Performance in Membrane Distillation

AbstractElectrospun nanofiber membranes (ENMs) have emerged as a cutting‐edge solution for membrane distillation (MD), recognized for their highly porous and interconnected architecture. This distinctive structure enables them to offer minimal mass transfer resistance, making them exceptionally suited for high‐efficiency membrane‐based separation processes. However, the very porosity that defines their strength also renders them vulnerable to fouling, scaling, and wetting during operation, which in turn compromises their performance. Current research efforts are geared toward overcoming these obstacles by refining the surface design and characteristics of ENMs. This review delves into the latest advancements in surface‐enhanced electrospun nanofiber membranes tailored for MD applications. It discusses the existing gaps in research and provides forward‐looking insights into the future of ENMs, spotlighting the development of membranes with precisely tunable surface attributes for optimized performance.

A MAX phase (Ti3AlC2) as a performance enhancer for poly(lactic acid) electrospun membranes in steam generation and solar desalination

Clean water and sanitation issues motivate researchers to develop water evaporators for freshwater generation. The composite membrane evaporator was electrospun herein based on poly(lactic acid) (PLA) and Ti3AlC2 MAX phase as a property enhancer. As a precursor for the MXenes synthesis, the MAX phase has never been explored with PLA for water evaporator potential. Alternative use of the MAX phase can reduce the production cost arising from chemical synthesis. This work explored the potential of the MAX phase as an additive to enhance PLA membrane performance for steam generation and desalination applications. Under the infrared irradiation (∼1.0 kW/m2), the mechanically-improved PLA/MAX phase membrane showed an enhanced water evaporation rate of 1.70 kg/m2 h (93.93 % efficiency), with an approximately 52 % rate increment relative to the PLA membrane. Based on the artificial seawater (3.5 % w/w), the membrane exhibited an evaporation rate of 1.60 kg/m2 h (87.57 % efficiency). The membrane showed self-floating ability at the air-water interface, excellent thermal stability over the entire operating temperatures, and reusability after repeated cycles. Moreover, the generated freshwater contained exceptionally low cations concentrations, as low as those in potable water. The developed composite membrane also had proved its potential for solar desalination in the water generation field.

Preparation and modification of PVDF membrane via VIPS method for membrane distillation

Membrane distillation (MD) is a promising technology for treating high-salinity mine water. However, MD membranes are known to have low membrane flux and are prone to fouling. In this study, we used the cost-effective and controllable vapor-induced phase separation (VIPS) technology to prepare polyvinylidene difluoride (PVDF) membranes, replacing the traditional immersion precipitation method and optimizing the membrane structure by including LiCl and acetone as porogen in the casting solution. The results showed that the membrane prepared using the VIPS method exhibited a highly open interconnected porous surface. Unlike traditional MD membranes with a dense epidermal layer and large finger-like pores, these optimized membranes had a symmetrical and uniform internal structure, leading to a high flux of 8.62 kg·(m2·h)−1 during direct contact membrane distillation testing. Different porogens produced varied results on the VIPS process and varying effects on membrane structure. The use of LiCl promoted the formation of PVDF β-phase, resulting in a decrease in the number of spherical nodules on the membrane surface, as well as improved density and smoothness. Consequently, this reduced fouling risk during membrane distillation while slightly decreasing membrane flux. On the other hand, acetone rapidly evaporated during the VIPS process, facilitating pre-gelatinization and α-phase formation of PVDF. This concurrent effect effectively restricted excessive nodule growth on the membrane surface, endowing the membrane with antifouling capabilities while preserving high flux.

Evaluating the feasibility of direct contact membrane distillation and nanofiltration in ground water treatment through a techno-economic analysis

This study delves into the realm of water treatment by conducting a comprehensive techno-economic evaluation of direct contact membrane distillation (DCMD) and nanofiltration (NF) processes. While previous research has explored the technical aspects of membrane distillation (MD) and nanofiltration, there remains a notable gap in economic analyses. Our research aims to bridge this gap by assessing the financial feasibility of employing MD and NF technologies for water desalination. Specifically, we scrutinize the performance of hydrophobic microporous flat sheet membranes crafted from polytetrafluoroethylene (PTFE) supported by non-woven polypropylene (PP) in desalinating brackish water through DCMD and NF processes. By varying operating conditions such as flow rate and feed temperature, we evaluate the membrane's efficacy. Employing an analytical model based on heat and mass transfer equations, we predict process performance across diverse scenarios. Our model demonstrates a high level of accuracy, with flux predictions deviating by less than 10% when utilizing the Knudsen-molecular mechanism model. Furthermore, through a detailed design and economic analysis of industrial-scale units for both processes, we reveal that the cost of permeated water is lower with NF compared to DCMD. Specifically, our calculations indicate a water cost of 1.34 USD/m3 for DCMD at a feed temperature of 65 °C with an 80% recovery rate, positioning it as a competitive option among conventional desalination methods. Notably, our financial assessment highlights that steam cost constitutes the primary expense in DCMD operations, contingent upon heating value and fuel prices. Noteworthy findings suggest that natural gas emerges as the most cost-effective fuel for steam production in a DCMD plant. This study underscores the economic viability and potential cost efficiencies associated with NF over DCMD in water treatment applications.

Towards the Optimization of a Photovoltaic/Membrane Distillation System for the Production of Pure Water.

The production of pure water plays a pivotal role in enabling sustainable green hydrogen production through electrolysis. The current industrial approach for generating pure water relies on energy-intensive techniques such as reverse osmosis. This study unveils a straightforward method to produce pure water, employing real-world units derived from previously simulated and developed laboratory devices. This demonstrated system is cost-effective and boasts low energy consumption, utilizing membrane distillation (MD) driven by the waste heat harnessed from photovoltaic (PV) panels. In a previous study, modeling simulations were conducted to optimize the multi-layered MD system, serving as a blueprint for the construction of prototype devices with a suitable selection of materials, enabling the construction of field-testable units. The most efficient PV-MD device, featuring evaporation and condensation zones constructed from steel sheets and polytetrafluoroethylene (PTFE) membranes, is capable of yielding high-purity water with conductivity levels below 145 μS with high flux rates.

Interactive effects of urine components and treatment conditions on antibiotic degradation of combined system integrating thermally activated peroxydisulfate and membrane distillation using machine learning

The antibiotic degradation in urine is gaining focus as it’s essential for resource recovery. Complex organic and inorganic substances in urine impact degradation efficiency through their involvement in radical-type chain reactions. In this study, more than 95 % of sulfamethoxazole (SMX) in urine was successfully degraded in both concurrent and sequential modes of a combined system integrating thermally activated peroxydisulfate and membrane distillation (TAP-MD), previously shown to be effective for resource recovery. Three algorithms, including random forest (RF), XGBoost, and support vector machine (SVM), were applied to model the impact of urine components and treatment conditions on SMX degradation. The XGBoot and RF models, fine-tuned by Bayesian optimization, are accurate and credible in both prediction and interpretation (R2 > 0.90). The models suggest that the improvement of SMX degradation by the MD process is due to PDS enrichment, with high PDS concentrations significantly promoting SMX degradation. Urea and HCO3− are the key factors impacting SMX degradation efficiency, followed by temperature, PDS, Cl−, and NH4+. Additionally, strong interaction effects between urine components were found on SMX degradation, contrasting with results from individual ions that were present in isolation. At high urea concentration (>100 mM), increasing concentrations of Cl−, NH4+, and HCO3− significantly inhibited SMX degradation. As the second key factor on SMX degradation, HCO3− can react with S2O82− without producing free radicals, resulting in a reduction in its yield. Furthermore, in the presence of high concentrations of HCO3−, the Cl− and NH4+ negatively affect the kobs of SMX degradation. The HCO3− competes with SMX for Cl to produce the weak oxidant CO3∙−, which reacts quickly with NH2∙ without producing other free radicals, causing a loss of free radicals.

A reduced model for pilot-scale vacuum-enhanced air gap membrane distillation (V-AGMD) modules: Experimental validation and paths for process improvement

A fast and robust computational model of a spiral-wound vacuum-enhanced air gap membrane distillation (V-AGMD) module at the pilot-scale is proposed and implemented. In contrast with data-driven models available in the literature, a physics-based approach is adopted for more reliable generalization beyond the validation dataset. A total of 86 experimental results, of which 41 are described in this work and 45 come from independent sources available in the literature, are used in the validation effort with quite favorable results. With the confidence on the robustness of the methodology due to the wide range of operational parameters and the use of spiral-wound modules of four different sizes included in the validation comparisons, a physical analysis is conducted varying the air gap pressure, the number of feedwater channels, the feedwater flow rate, and the membrane area. Improvements of up to 60% in both water productivity and energy efficiency can be achieved by intensifying the vacuum in the air gap or decreasing the number of feedwater channels. These parameters achieve performance gains due to less resistance in the air gap for vapor to migrate through it, in the former case, and a reduced temperature polarization effect, in the latter case. Smaller flow rates favor energy efficiency at the expense of water productivity by simultaneously decreasing transport-phenomena-related irreversibility and the partial pressure difference across the membrane and the air gap. In addition, this tradeoff between energy efficiency and driving force is shown to lead to an optimum value for the membrane area beyond which the permeate flow rate through the membrane starts to fall due to the small driving force. An illustrative case is predicted to achieve energy efficiency metrics, such as a gain-output ratio of 12.7, competitive with multi-effect distillation.

Integration of proton exchange membrane fuel cell with air gap membrane distillation for sustainable electricity and freshwater cogeneration: Performance, influential mechanism, multi-objective optimization and future perspective

Proton exchange membrane fuel cell (PEMFC) converts vast majority of hydrogen energy into waste heat, resulting in energy wasting, membrane drying and even lifetime shortening. Herein, air gap membrane distillation is proposed to immediately remove and harness the waste heat from PEMFC for additional freshwater production. By quantifying the irreversible thermodynamic and electrochemical losses, mathematical formulas for the water production, power generation and overall efficiency are derived to evaluate the system performance. After a rigorous model validation, the performance feature, feasibility and competitiveness of the proposed system are examined. The integration system achieves a peak power density 23.69 % higher than that of a single PEMFC at 353 K, with a corresponding increase in exergy efficiency by 3.61 %. In addition, the influential mechanisms of the PEMFC operating conditions, air gap thickness, coolant temperature, and properties of the proton exchange membrane and hydrophobic porous membrane are investigated to discern potential avenues for further performance improvement. The gradient-based local sensitivity analyses further determine the optimal parameter regulation strategies for different output targets. The economic study indicates that the levelized costs of water and electricity over the total lifecycle are 21.53 $ m−3 and 0.061 $ kWh−1, respectively. The multi-objective optimization, integrating genetic algorithm and the technique for order preference by similarity to an ideal solution, maximizes the exergy efficiency and output power density while minimizing the initial investment cost per unit area, with values of 42.84 %, 0.975 W cm−2, and 27.33 $ cm−2, respectively.